a) Field of the invention
The present invention relates to an attachment lens system which is to be used in combination with photographic lens systems for cameras.
b) Description of the prior art
In the recent days, zoom lens systems are widely used as photographic lens systems for photographic cameras, video cameras and so on. However, the zoom lens systems have a common defect that they must be composed of large numbers of lens elements for correcting aberrations and therefore have large external designs, thereby enlarging the cameras which are to comprise these zoom lens systems.
On the other hand, it is possible to design a compact camera by using a photographic lens system which has a fixed focal length. However, such a lens system has another defect that it has only one field angle, thereby limiting photographing variety.
The defects described above can be corrected, or a great photographing variety can be obtained with a compact photographic lens system, by equipping a camera with a master lens system having a fixed focal length and combining it with an attachment lens system which permits varying focal length of the photographic lens system as occasion demands.
As an example of attachment lens systems which are designed for use at the tele position, there is known the attachment lens system disclosed by Japanese Patent Kokai Publication No. Hei 1-251009. This attachment lens system is designed, as shown in FIG. 1B, as an afocal lens system which comprises, in the order from the object side, a lens unit having a positive refractive power and another lens unit having a negative refractive power, and is to be arranged before a master lens system, at the location enclosed by the dashed line in FIG. 1A, for permitting variation of focal length of the master lens system.
Further, as an example of attachment lens systems to be used at the wide position, there is known the lens system proposed by Japanese Patent Kokai Publication No. Hei 1-319010 which is designed as an afocal lens system comprising, in the order from the object side, a lens unit having a negative refractive power and another lens unit having a positive refractive power.
However, each of these conventional examples is an attachment lens system which has a fixed focal length and a magnification within a range of approximately from 0.5.times. to 3.0.times., and permits the photographing variety within a certain limited range.
Furthermore, it is possible to obtain a higher magnification by using an afocal lens system of the Keplerian type as an attachment lens system. However, the attachment lens system of this type poses a problem that it extremely prolongs the total length of the photographic lens system which is composed by combining the attachment lens system with a master lens system.
In addition, there are known the attachment lens systems which are designed as afocal zoom lens systems each of which consists of three lens units, i.e., a positive lens unit, a negative lens unit and a positive lens unit, as exemplified by the lens system disclosed by Japanese Patent Kokai Publication No. Sho 53-27044. However, this attachment lens system has a low zooming ratio of approximately 2 which is not sufficient for enhancing the photographing variety.